1. Field of the Invention
The present invention is directed to a tissue-optical measuring arrangement for the examination of a preferably living subject with visible, NIR or IR light, i.e. the examination is carried out in vivo preferably. The wavelength of the visible light thereby lies between 380 and 780 nm, that of NIR light (near infrared light) lies between 780 nm and 1.5 .mu.m and that of IR light (infrared light) lies between 1.5 .mu.m and 1 mm, whereby it is particularly the range from 1.5 .mu.m through 15 .mu.m that is of significance in the present invention given the employment of IR light.
2. Description of the Prior Art
Many optical properties of tissue such as, for example, the absorption, the scattering or dispersion and the spectral properties can be identified by directing light of the aforementioned wavelength ranges at a region of the subject. For example, it is possible to identify tissue modifications in mammary diagnostics or to acquire information about the blood supply of the brain in pediatrics and/or neurology by directing light of the aforementioned wavelength ranges, for example at a mammary gland or a skull, with the light emerging from the subject being detected and the information acquired in this way being interpreted by a suitable technique. It is thereby advantageous that these are usually non-invasive procedures. Further details of examination of this type are discussed, can be derived, for example, in the publications "Cerebral Oxygenation Measuring System NIR-100" (Tentative Data), Hamamatsu Photonics K. K., System Division, September 1987; "Optical Spectroscopy", Robert L. Egan et al., Acto Radiologica, Vol. 29, Fasc. 5, September-October 1988; "Cerebral Monitoring in Newborn Infants by Magnetic Resonance and Hear Infrared Spectroscopy", D. T. Delpy et al., Departments of Medical Physics and Bioengineering, Pediatrics and Physiology, University College London. Unfortunately, the light emerging from the subject that is to be detected, which can be back-scattered (diffusely reflected) light or dispersed transmitted light, contains information about the entire region of the subject illuminated with the incoming light. The measurement is thus not location-selective. When detecting the back-scattered light, this means that, in particular, one does not know at what depth the light was reflected, i.e. the distance from the surface of the subject measured parallel to the propagation direction of the incident light. This is especially disturbing when one wishes to examine the optical properties of a subject at a specific depth. One must then simultaneously measure most of the light back-scattered from the surface of the subject and the surface-proximate regions thereof. This leads to a poor signal-to-noise ratio of the measurement and can even lead to unusable results of the measurement beginning at a specific thickness of the subject. For measurements wherein the light transmitted through the subject is detected, the lack of location-selectivity means that the path taken by the light transmitted through the subject cannot be specifically identified. Again, the signal-to-noise ratio deteriorates with increasing thickness of the subject, to the point that the measured results are unusable.
Heretofore, only one method fundamentally suitable for employment in vivo was known which, however, only comes closer to resolving the aforementioned problems with extremely great outlay. This method is described in the article "Estimation of Optical Pathlength through Tissue from Direct Time of Flight Measurement", D. T. Delpy et al., Phys. Med. Biol., 1988, Vol. 33, No. 12, pages 1433-1442 and is based on the time of flight measuring principle using a pulsed laser as a light source and an ultra-fast streak camera as a detector. The pulse duration of the laser is typically less than 1 picosecond. The chronological resolution of the streak camera lies on the order of magnitude of approximately 2 picoseconds. Since the light is back-scattered from the subject to be examined in different depths, penetrates the subject on different paths, the individual parts of the back-scattered or transmitted light have different arrival times at the streak camera. The detected light parts can thus be selected and detected according to arrival time, and thus according to the depth in the subject from which they were back-scattered, or the path which they took through the subject. A time of flight measuring system having an adequate chronological resolution, and thus an adequate topical resolution, however, is expensive.